The present invention relates generally to stents which are implantable or deployable in a vessel or duct within the body of a patient to maintain the lumen of the duct or vessel open, and more particularly to improvements in stent coatings and in methods for applying such coatings.
Stents are expandable prostheses employed to maintain narrow vascular and endoluminal ducts or tracts of the human body open and unoccluded, such as a portion of the lumen of a coronary artery after dilatation of the artery by balloon angioplasty, for example. The stent itself has the basic form of an open-ended tubular element with openings through a side thereof, which is adapted to be expanded from a first outside diameter sufficiently small to allow the stent and its delivery system to traverse the vascular system of the human body to reach a site in the blood vessel at which the stent is to be deployed, to a second outside diameter sufficiently large to engage the inner lining of the vessel for retention at the site.
In the exemplary case of an occluded coronary artery, the original blockage typically arises from a buildup of fatty deposits or plaque on the inner lining of the vessel. The balloon angioplasty procedure is used to compress the deposits against the inner lining of the vessel, or virtually entire removal may be achieved using other types of angioplasty such as laser or rotational cutting. A different mechanism, however, may cause a new blockage after the angioplasty procedure is performed. The blood vessel wall is subjected to trauma by the balloon, laser or rotating knife, as the case may be, which results in intimal hyperplasia, i.e., a rapid proliferation of smooth muscle cells in the affected region of the wall, to cause restenosis and re-occlusion of the vessel lumen in a significant percentage of angioplasty patients within a period of from three to six months following the initial procedure.
To avoid this re-occlusion and to maintain the lumen of the vessel open, it is now customary procedure to install a stent at the site in the vessel where the angioplasty was performed. The stent is deployed by radial expansion under pressure exerted, for example, by active inflation of a balloon of a balloon catheter on which the stent is mounted, or in some instances by passive spring characteristics of a pre-formed elastic stent, to engage the inner lining or inwardly facing surface of the vessel wall with sufficient resilience to allow some contraction but also with sufficient stiffness to resist to a great degree the natural recoil of the vessel wall that follows its expansion.
The stent itself, however, offers a surface that can promote thrombus formation as blood flows through the vessel. This can result in an acute blockage, which, in a coronary artery, is sufficient to produce an infarction. The thrombosis and clotting can be reduced or even eliminated by localized application of appropriate drugs in a biodegradable formulation, which act for only a period of time sufficient to stave off the thrombus reaction to the presence of the stent in the bloodstream. Some difficulty is encountered in providing a stent surface which is suitable for retention of the necessary drug(s) to achieve those purposes.
A similar situation is encountered at the outward facing surface of the stent that contacts and engages the inner lining of the vessel, duct or tract, where tissue irritation can exacerbate fibrosis of the vessel wall and restenosis in the region of the irritation. Here, also, it would be desirable to provide the stent with the capability to provide a timed release of suitable drug(s) from a biodegradable carrier on or in the affected stent surface, to reduce the occurrence of fibrosis and hyperplasia at the portion(s) of the vessel wall contacted by the stent.
Another factor affecting the decision to implant a stent is the ability of the patient to tolerate the presence of the material of which the stent is composed. Here again, biomaterial coatings can be helpful. But a statistically significant five percent of the patient population is allergic to materials of which currently available stents are composed, including chrome, nickel, and/or medical grade implantable 316L stainless steel, which contains about 20% nickel. In such patients, stent implants are contraindicated and may be used only for acute relief on an emergency basis. Indeed, restenosis can commence within only one or two days to very quickly eliminate the initial benefit enjoyed from implantation of a conventional stent.
An additional need encountered for stent usage in the human body include a capability to clearly visualize the stent by means of X-ray fluoroscopy as it is being implanted at the preselected site in the body, as by advancement on a stent delivery system through a portion of the patient's vascular system and into a coronary artery, and after the stent is implanted, for purposes of its examination from time to time at the implant site. This generally requires that the wall of the stent be of sufficient thickness to not only withstand the aforementioned vessel wall recoil phenomenon, but to enable it to be seen on the fluoroscope.
Several materials possess the mechanical strength which is suitable for use in a stent structure, one which is particularly effective being 316L stainless steel. Typical stent wall or wire thicknesses range from 70 to 200 microns (or micrometers, em). A 70 to 80 .mu.m thick 316L steel stent, clearly at the lower end of this range, has been found to provide sufficient strength to resist recoil and to maintain a lumen diameter very nearly corresponding to that achieved by the balloon inflation. But this relatively thin and tiny metal structure creates little shadow on a fluoroscopic picture, since the X-ray absorption of the metal is low. On the other hand, a need for greater wall thickness to enhance radiopacity must be balanced against an increased stent diameter which makes passage through narrow vessels more difficult and risky, as well against a requirement of sufficient radial force to be applied by balloon inflation on the interior surface of the stent during deployment, with concomitant increased risk of balloon rupture.
Among the most important features of a suitable stent are the following. The device should be flexible, and yet possess sufficient mechanical strength to resist vessel recoil. It should demonstrate a high rate of successful interventional placement, be highly visible on x-ray fluoroscopy, be very thin to minimize obstruction by its mere presence in the lumen intended to be dilated and held open, and not be an agent which promotes a re-narrowing or re-occlusion of the vessel or duct lumen in which it is implanted. Stent design, of course, can play a major role in influencing the aforementioned features, but also significant is the material(s) of which the stent is composed, with respect to visibility, flexibility, and recoil-resistant characteristics of the stent, as well as its surface characteristics that affect capability of the stent to prevent or inhibit thrombus formation and restenosis in a blood vessel in which the stent is implanted. Current stents have not proved to be capable of fulfilling all of these requirements.
Aside from vascular usage, other ducts or tracts of the human body in which a stent might be installed to maintain an open lumen include the tracheo-bronchial system, the biliary hepatic system, the esophageal bowel system, and the urinary tract system. Many of the same requirements are found in these other endoluminal usages.
Therefore, among the principal aims of the present invention are to provide a stent which has a composition that offers an enhanced capability to fulfill these important requirements, as well as which enables optimum mechanical support of the vessel despite a thin stent diameter and low profile while still offering optimum fluoroscopic appearance so that the position of the stent within the vascular system is readily identified.
Another aim of the invention is to provide such a stent which possesses farther properties and characteristics to deter restenosis even without biomaterial coatings, as well as avoid allergic reaction of the patient to the presence of the implanted stent.